The respiration of a variety of bulky plant storage organs is sharply stimulated by cyanide. Isolated mitochondria from such tissues are normally cyanide sensitive, as are fresh slices. The respiration of slices aged some 24 hours returns to a cyanide-in-sensitive state, mitochondria prepared therefrom again manifesting cyanide sensitivity. A marked departure from the foregoing behavior is found in yam, where the respiration of fresh slices and isolated mitochondria is stimulated by cyanide much as is that of the intact organ. The cyanide resistant, or alternate, electron path is thought to branch from the conventional electron path on the substrate side of cytochrome b. It is proposed to study the basis of the engagement of the alternate path, with particular emphasis on the question of whether CN insensitivity follows upon the activation per se of the alternate path, or is the result of the establishment of a bridge between the conventional and alternate pathways. Particular interest centers on the possible role of ethylene in implementing the switch from one path to another, since ethylene and cyanide stimulate respiration and decontrol glycolysis to the same extent in a great variety of different fruits and storage organs. On the one hand, it will be attempted to prepare CN-insensitive mitochondria from potato where insensitivity is normally lost during isolation, and on the other to convert CN-insensitive yam mitochondria to a CN-sensitive condition. Tissue and organ studies will be continued to investigate the influence of the operation of the CN-insensitive path on associated phenomena such as sweetening and ripening.